Some passive sonar systems are designed to receive and process underwater sound signals emitted by an unknown water born vessel. The sound signals can be active sound pulses emitted by an active sonar system on board the unknown vessel, or vessel noise (e.g., engines, generators, and the like). The passive sonar systems can receive a combination of ambient ocean noise and the vessel-generated sound signals. The passive sonar systems can employ a variety of processing upon the received sound signals in order to detect, to localize, and to classify the unknown vessel.
Some sounds in the water tend to be amplitude modulated by the sound field emitted by the vessel's propellers. In particular, the sound received by the passive sonar system can be amplitude modulated in a manner related to characteristics of the propeller.
Some passive sonar systems have exploited the amplitude modulation of the received sound in order to identify characteristics of the propeller, for example, rotation speed and number of propeller blades. With this information, the passive sonar systems are often able classify the type of vessel, including, but not limited to, whether the vessel is a surface vessel or a submarine. The processing can be of a type referred to as “detection of envelope modulation on noise.”
Referring to FIG. 1, a conventional “detection of envelope modulation on noise” system 10 includes a hydrophone 14 adapted to receive underwater sound 12. The hydrophone 14 is conventionally an omnidirectional hydrophone, which has substantially the same sensitivity to sound received from all spatial directions. The hydrophone 14 generates a signal in response to the sound signal 12. The signal is preprocessed, for example, by an amplifier 16. The amplifier 16 is coupled to an analog to digital (A/D) converter 18, which generates a signal x(t), which is comprised of digital time samples of the preprocessed signal.
The signal x(t) can be processed to identify the above-described amplitude modulation of the received sound 12. One of ordinary skill in the art will recognize a variety of circuits that can be used to identify the amplitude modulation of the received sound 12. In one conventional arrangement, the signal x(t) can be processed by a “square law” detector, including a squaring module 20 and a low pass filter (LPF) module 22. An output signal generated by the low pass filter 22 is representative of the envelope of (i.e., the amplitude modulation of) the received sound signal 12.
The output signal generated by the low pass filter module 22 can be analyzed by a spectrum analyzer 24, for example, a Discrete Fourier Transform (DFT). It will be understood that the spectrum analyzer 24 provides a frequency domain signal (e.g., one or more frequency spectra) representative of frequency content of the envelope of the received sound signal 12. The frequency spectra generated by the spectrum analyzer 24 can be further processed and displayed by a detector/display module 26. For example, the detector/display module 26 can display the frequency spectra in a waterfall type display (not shown). The detector/display module 26 can also detect and analyze spectral lines present in the frequency spectra.
It is possible to determine a propeller shaft rate (revolutions per second (rps)) and a number of propeller blades of a detected vessel by analyzing the frequency spectra. From the shaft rate and the number of propeller blades it is often possible to identify the type of vessel and whether the vessel is a surface vessel or a submarine.
In general, a fundamental frequency of the frequency domain signal (frequency spectra) generated by the spectrum analyzer 24 in Hz corresponds to the propeller shaft rate of the unknown vessel in revolutions per second. Furthermore, the number of propeller blades can be determined from frequencies and relative amplitudes of harmonic signal components in the frequency domain signal generated by the spectrum analyzer 24.
The “detection of envelope modulation on noise” system and methods described above are often able to detect and to classify a vessel. However, in general, it is always desirable to improve detection performance, localization performance, and/or classification performance of a sonar system.